Rocky Super-Earth and Gas Giant Are Latest Superstar Couple

A rocky planet 1.5 times the size of Earth and a Neptune-like gas giant were spotted orbiting a star daringly close to each other.

The two-planet Kepler-36 system is the Kepler space telescope’s latest treasured find, reported today in the journal Science by researchers from the Harvard-Smithsonian Center for Astrophysics and the University of Washington. Passing less than five Earth-moon distances apart every 97 days, the planets are far closer together and more different in density than any two planets in our solar system.

Looking from the smaller planet’s surface (as depicted above), the gas giant would appear more than twice as big as the full Moon appears from Earth. The wrenching gravity between the planets would stretch and squeeze them, which might make the rocky one volcanic.

The Kepler telescope found the pair of orbs by detecting mini-eclipses of their star as they crossed in front of it, dimming its light. A year on these planets, or one orbit, is just 14 days for the rocky planet and 16 days for the gas giant. To find out the masses and radii of the planets, astronomers measured the oscillating dance of their star, caused by sound vibrations in its stellar atmosphere. Then, they calculated the planets’ sizes using a model based on the amount of light blocked by the planets.

The close spacing of this odd planetary duo, with such contrasting composition and densities, has the scientists stumped. The planets’ orbits might have started further apart and migrated closer together. Or the planets themselves might have had compositional makeovers, as stellar radiation ate away at their atmospheres. Add it to a list of oddities in the Kepler mission’s planetary menagerie.